Systems and methods for power and flow rate control

ABSTRACT

The invention is generally directed to systems and methods for fluid control, and more particularly to systems and methods for power and flow rate control for aspiration. In accordance with one embodiment, an aspiration system includes an aspiration line having distal and proximal ends and an aspiration port defined in the distal end; a fluid transport device operatively coupled to the proximal end of the aspiration line; and a flow restrictor operatively coupled to the aspiration line in between the fluid transport device and the aspiration port. To measure occlusion within the line, first and second pressure sensors are utilized, the first sensor being operatively coupled to the aspiration line between the port and the restrictor and the second sensor being operatively coupled to the aspiration line between the restrictor and the fluid transport device. The pressure differential between the two sensors can provide an indication of the onset, presence, and/or elimination of an occlusion.

FIELD OF THE INVENTION

The field of the invention relates to systems and methods for fluidcontrol, and more particularly to systems and methods for power and flowrate control.

BACKGROUND OF THE INVENTION

A number of medically recognized techniques are utilized for cataracticlens removal based on, for example, phacoemulsification, mechanicalcutting or destruction, laser treatments, water jet treatments, and soon.

The phacoemulsification method includes making a corneal incision andthe insertion of a phacoemulsification handpiece which includes a needlethat is ultrasonically driven in order to emulsify, or liquefy, thelens. A phacoemulsification system 5 known in the art is shown inFIG. 1. The system 5 generally includes a phacoemulsification handpiece10 coupled to an irrigation source 30 and an aspiration pump 40. Thehandpiece 10 includes a distal tip 15 (shown within the anterior chamberof the patient's eye 1) that emits ultrasonic energy to emulsify thecataractic lens within the patient's eye 1. The handpiece 10 furtherincludes an irrigation port 25 proximal to the distal tip 15, which iscoupled to an irrigation source 30 via an irrigation line 35, and anaspiration port 20 at the distal tip 15, which is coupled to anaspiration pump 40 via an aspiration line 45. Concomitantly with theemulsification, fluid from the irrigation source 30, which is typicallyan elevated bottle of saline solution, is irrigated into the eye 1 viathe irrigation line 35 and the irrigation port 25, and the irrigationfluid and emulsified cataractic lens material are aspirationd from theeye 1 by the aspiration pump 40 via the aspiration port 20 and theaspiration line 45. Other medical techniques for removing cataracticlenses also typically include irrigating the eye and aspirating lensparts and other liquids. Additionally, some procedures may includeirrigating the eye 1 and aspirating the irrigating fluid withoutconcomitant destruction, alteration or removal of the lens.

Aspiration can be achieved with a variety of different aspiration pumps40 known in the art. The two most common types are (1) volumetric flowor positive displacement pumps (such as peristaltic or scroll pumps) and(2) vacuum-based pumps (such as venturi, diaphragm, or rotary-vanepumps). Each type has its own general advantages and disadvantages.Turning to FIG. 2, an example peristaltic flow pump 50 is illustrated.In this configuration, the aspiration line 45 is in direct contact witha rotating pump head 50 having rollers 52 around its perimeter. As thepump head 50 rotates clockwise, the rollers 52 press against the line 45causing fluid to flow within the line 45 in the direction of the rollers52. This is referred to as a volumetric flow pump because the pump 50directly controls the volume or rate of fluid flow. An advantage withthis type of pump 50 is that the rate of fluid flow can be easily andprecisely controlled by adjusting the rotational speed of the pump head50.

Turning to FIG. 3, an example vacuum-based pump 60 is illustrated. Thistype of pump indirectly controls fluid flow by controlling the vacuumwithin the fluidic circuit. For example, the vacuum-based pump 60 can bean pneumatic pump (e.g., a venturi pump) that creates a pressuredifferential in a drainage cassette reservoir 65 that causes the fluidto be sucked from the aspiration line 45 into the drainage cassettereservoir 65. Thus, instead of pushing fluid through the aspiration line45 like the flow pump 50, the fluid is essentially pulled by vacuumthrough the line 45. The rate of fluid flow generated by a vacuum-basedpump is generally higher than the rate of fluid flow generated by avolumetric flow based pump; however, current systems and methods forcontrolling the rate of volumetric flow for the vacuum-based pump, whichtypically involve adjusting the operative vacuum level, are imprecise,which raises safety and efficacy concerns.

As is well known, for these various surgical techniques it is necessaryto maintain a stable volume of liquid in the anterior chamber of the eyeand this is accomplished by irrigating fluid into the eye at the samerate as aspirating fluid and lens material. For example, see U.S. Pat.No. 5,700,240, to Barwick et. al, filed Jan. 24, 1995 (“Barwick”) andU.S. patent application Ser. No. 11/401,529 to Claus et. al, filed Apr.10, 2006 (“Claus”), which are both hereby incorporated by reference intheir entirety. During phacoemulsification, it is possible for theaspirating phacoemulsification handpiece 10 to become occluded. Thisocclusion is caused by particles blocking a lumen or tube in theaspirating handpiece 10, e.g., the aspiration port 20 or irrigation port25. In the case of volumetric flow based pumps, this blockage can resultin increased vacuum (i.e. increasingly negative pressure) in theaspiration line 45 and the longer the occlusion is in place, the greaterthe vacuum. In contrast, with a vacuum-based pump, this blockage canresult in a volumetric fluid flow drop off near the aspiration port 20.In either case, once the occlusion is cleared, a resulting rush of fluidfrom the anterior chamber into the aspiration line 45 can outpace thevolumetric flow of new fluid into the eye 1 from the irrigation source30.

The resulting imbalance of incoming and outgoing fluid can create aphenomenon known as post-occlusion surge or fluidic surge, in which thestructure of the anterior chamber moves rapidly as fluid is replaced dueto the dynamic change in the rate of fluid flow and pressure. Suchpost-occlusion surge events may lead to eye trauma. The most commonapproach to preventing or minimizing the post-occlusion surge is toquickly adjust the vacuum-level or rate of fluid flow in the aspirationline 45 and/or the ultrasonic power of the handpiece 10 upon detectionof an occlusion. Many surgeons rely on their own visual observations todetect the occlusion; however, because of the unpredictable andtime-sensitive nature of the problem, a reliable computer-baseddetection and response system is preferable.

For current systems with volumetric flow pumps 50, if an occlusionoccurs, the flow rate will decrease at the aspiration port 20 and thevacuum level within the aspiration line 45 between the pump 50 and thehandpiece 10 will increase. Thus, a computer-based system (not shown)can utilize a vacuum sensor 55 placed on the aspiration line 45 todetect the vacuum increase and respond accordingly (an example of such asystem is described in “Barwick” and “Claus”). For current systems withvacuum-based pumps 60, however, the vacuum level within the aspirationline 45 is tied to the vacuum power generated by the pump 60 and thus,may not be an effective indicator of whether an occlusion has occurred.Accordingly, an improved system and method for controlling the rate offluid flow and vacuum based on the detection of occlusion within a fluidcircuit is desirable.

SUMMARY OF THE INVENTION

The invention is generally directed to systems and methods for fluidcontrol, and more particularly to systems and methods for power and flowrate control for aspiration.

In accordance with one embodiment, an aspiration system includes anaspiration line having distal and proximal ends and an aspiration portdefined in the distal end; a fluid transport device operatively coupledto the proximal end of the aspiration line; and a flow restrictoroperatively coupled to the aspiration line in between the fluidtransport device and the aspiration port. To measure occlusion withinthe line, first and second pressure sensors are utilized, the firstsensor being operatively coupled to the aspiration line between the portand the restrictor and the second sensor being operatively coupled tothe aspiration line between the restrictor and the fluid transportdevice. The pressure differential between the two sensors can provide anindication of the onset, presence, and/or elimination of an occlusion.

In accordance with another embodiment, the flow restrictor is a variableflow restrictor controllable by a computer processor.

Other systems, methods, features and advantages of the invention will beor will become apparent to one with skill in the art upon examination ofthe following figures and detailed description. It is intended that allsuch additional systems, methods, features and advantages be includedwithin this description, be within the scope of the invention, and beprotected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better appreciate how the above-recited and other advantagesand objects of the inventions are obtained, a more particulardescription of the embodiments briefly described above will be renderedby reference to specific embodiments thereof, which are illustrated inthe accompanying drawings. It should be noted that the components in thefigures are not necessarily to scale, emphasis instead being placed uponillustrating the principles of the invention. Moreover, in the figures,like reference numerals designate corresponding parts throughout thedifferent views. However, like parts do not always have like referencenumerals. Moreover, all illustrations are intended to convey concepts,where relative sizes, shapes and other detailed attributes may beillustrated schematically rather than literally or precisely.

FIG. 1 is a diagram of a phacoemulsification system known in the art.

FIG. 2 is a diagram of a phacoemulsification system having a flow pumpknown in the art.

FIG. 3 is a diagram of a phacoemulsification system having avacuum-based pump known in the art.

FIG. 4 is a diagram of a vacuum-based aspiration system in accordancewith a preferred embodiment.

FIG. 5 a is a diagram of a variable flow restrictor in accordance with apreferred embodiment.

FIG. 5 b is a diagram of another variable flow restrictor in accordancewith a preferred embodiment.

FIG. 6 is a diagram of another vacuum-based aspiration system inaccordance with a preferred embodiment.

FIG. 7 is a diagram of another vacuum-based aspiration system inaccordance with a preferred embodiment.

FIG. 8 is a diagram of a phacoemulsification system in accordance with apreferred embodiment.

FIG. 9 is a diagram of an irrigation/aspiration system in accordancewith a preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

What are described below are preferred embodiments of aspiration systemsusing vacuum-based pumps, which can be applied to any system, medical(such as phacoemulsification, wound drainage, blood circulation,dialysis, or similar), or non-medical.

Variable Flow Restrictors

In general, the rate of fluid flow is a function of three (3) basicparameters: (1) the effective viscosity of the fluid, (2) the operativeor driving vacuum level, and (3) the instantaneous effective resistanceof the fluid transport pathway (e.g., aspiration line 45). For manysurgical procedures, a constant effective viscosity can be assumed(particularly if the fluid is predominantly a known irrigation source),and further, the operative or driving vacuum level can be easilymeasured and adjusted in an open-loop pressure monitoring system;however, for current aspiration systems with vacuum based pumps, theinstantaneous effective resistance of the fluid transport pathway is aparameter that can vary independent of the vacuum-level, and thus, isnot easily quantified or dynamically compensated for in cases such asfluid transport pathway obstruction (e.g., occlusion), which is asignificant reason why volumetric flow rate is difficult to control byonly adjusting the operative vacuum-level.

To compensate for the third parameter, one existing approach is toutilize a fixed flow restrictor (not shown) on the aspiration line 45between the aspiration port 20 and the pump 60. The fixed flowrestrictor can be an orifice within the line 45 that reduces thecross-sectional area of the portion of the line 45 having the orifice.This, in turn, increases the instantaneous effective resistance of theline 45 and reduces the volumetric flow rate. These fixed flowrestrictors typically drop the volumetric flow rate down to ratesgenerated by flow pumps 50, which can create a safer operatingenvironment for surgeons (e.g., suppress post-occlusion surge), butthese fixed flow restrictors also undesirably limit the range ofoperation that a vacuum-based pump can provide (e.g., range of allowablevolumetric flow rates).

One approach to address this issue is to utilize a variable flowrestrictor. Turning to FIG. 4, a vacuum-based aspiration system 100having a variable flow restrictor 150 is shown. The variable flowrestrictor 150 is located on an aspiration line 110 having an aspirationport 120 at its distal end, and a drainage cassette 130 and vacuum pump140 at its proximal end. The variable flow restrictor 150 is generally adevice, preferably controllable by a computer system (not shown), thatvariably controls the instantaneous effective resistance of the fluidpathway (e.g., the aspiration line 110) of the system 100. This canallow for precise control of the volumetric flow rate for a vacuum-basedpump 140 while still utilizing a wide range of operation. In a preferredembodiment, the variable flow restrictor 150 is configured to deform aspecific, localized, deformable segment 115 of the aspiration line 110.By distorting the cross-sectional area of the segment 115 into a smallertotal area or by significantly distorting the width vs. height ratio ofthe segment 115, the instantaneous effective resistance can beincreased, which inversely lowers both the current actual volumetricflow rate and also the theoretical maximum volumetric flow ratepotential of the fluid.

Turning to FIG. 5 a, an example variable flow restrictor 200 is shown inmore detail. The segment 115 of the aspiration line 110 coupled to thevariable flow restrictor 200 is preferably made of an elastomericdeformable tubing having a resistance variability of at least 2:1(comparing non-deformed vs. maximum deformation cross-sectionalprofiles). The variable flow restrictor 200 includes a plunger 210coupled to an actuator 230 controllable by a computer system (notshown). During operation, the actuator 230 pushes the plunger 210 intomechanical contact with the deformable segment 115 of the aspirationline 110, causing the segment 115 of the aspiration line 110 to deform,thereby adjusting the effective resistance, as explained above. Theactuator 230 can be any type of actuator known in the art, such as amechanical actuator (e.g., a linear motor, axial solenoid, rotarysolenoid, or electro magnetic motor), a pneumatic actuator (e.g., suchas a low friction pneumatic rotary or axial bladder/cylinder with avariable pressure supply) or a thermal actuator (e.g., such as abi-metallic strip). A pneumatic actuator is preferable because it can becontinuously variable, which desirably increases the resolution of thecontrol of the plunger 210 and thus the control of the effectiveresistance. In addition, a spacer 220, such as a wedge or cam, (alsocontrollable by a computer system (not shown)), can be utilized inbetween the plunger 210 and the actuator 230 (or behind the actuator240) to further control the plunger's 210 range of motion, therebyincreasing controllability of the effective resistance.

Turning to FIG. 5 b, another variable flow restrictor 300 is showncoupled to the aspiration line 110 (shown in cross-sectional view). Thisvariable flow restrictor 300 includes a rigid case 350, having a fixedheight, surrounding the line 110. The case 350 limits the amount ofdeformation on the line 110. The restrictor 300 further includes a flatplunger 310 having a width greater than the width of the case 350. Theplunger 310 is coupled to an actuator (not shown) such as the actuator230 described above. The actuator pushes the plunger 310 into mechanicalcontact with the line 110, causing the line 110 to deform; however,deformation of the line 110 is limited to the top of the case 350.

Other variable flow restrictors 150 that deform segment 115 includedevices that stretch the segment 115 in the longitudinal direction ofthe line 110 (not shown) and also devices that twist the segment 115into a spiral shape (not shown). In addition, the variable flowrestrictor 150 can be a device that is integrated with the line 110, forexample, a chamber (not shown) within the line 110 with a deformablecross-section.

Flow Rate Sensing Method

As mentioned above, with aspiration systems having flow pumps 50 (FIG.2), if an occlusion occurs, the vacuum level within the aspiration line45 between the pump 50 and the aspiration port 20 will increase;however, for current systems with vacuum-based pumps 60 (FIG. 3), thevacuum level within the aspiration line 45 is tied to the vacuumgenerated by the pump 60 and thus, may not be an effective indicator ofwhether an occlusion has occurred. Turning to FIG. 6, anothervacuum-based aspiration system 1000 is shown. The aspiration system 1000includes a flow restrictor 1200, which is preferably a variable flowrestrictor but can be a fixed flow restrictor, located on an aspirationline 1110 having an aspiration port 1120 at its distal end and adrainage cassette 1130 and vacuum-based pump 1140 at its proximal end.Further included are first and second vacuum sensors 1300/1350. Thefirst vacuum sensor 1300 is operatively coupled to the aspiration line1110 between the port 1120 and the restrictor 1200 (“port 1120 side’),and the second vacuum sensor 1350 is located between the restrictor 1200and the pump 1140 (“pump 1140 side”).

As one of ordinary skill in the art would appreciate, during aspiration,by increasing the effective resistance in a localized segment of theaspiration line 1110, the flow restrictor 1200 will produce adifferential volumetric flow rate between the port 1120 side of the lineand the pump 1140 side of the line. This accordingly, will cause avacuum or pressure differential, ΔP, between the port 1120 side of theline 1110 and the pump 1140 side of the line. The vacuum level, orpressure, on the pump 1140 side of the line 1110 will generally besubstantially tied to the vacuum level of the pump 1140; however, thevacuum level, or pressure, on the port 1120 side can vary with thevolumetric flow rate in the port 1120 side. For example, if an occlusionin the port 1120 occurs, the volumetric flow rate on the port 1120 sideof the line will be reduced, which will in turn reduce the pressure,P_(port-side), on the port 1120 side of the line, while the vacuum, orpressure, P_(pump-side), on the pump 1140 side of the line remainssubstantially tied to the vacuum-level of the pump. By utilizing thefirst and second vacuum sensors 1300/1350, a ΔP(P_(port-side)−P_(pump-side)) pressure differential can be measured andutilized in a computer-based algorithm, such as those described in theClaus and Barwick applications referenced above, to detect the onset,presence, breakage, or elimination of an occlusion. If the flowrestrictor 1200 is a variable flow restrictor, then the vacuum-basedaspiration system 1000 can provide both computer-based detection ofocclusion and precise control of the volumetric flow rate while stillmaintaining the vacuum-based pump's 1140 full range of operation.

Another approach to detect the occurrence of an occlusion is to utilizea Doppler flow meter, known in the art, to measure the volumetric flowrate in the aspiration line (not shown). When an occlusion occurs, thevolumetric flow rate will decrease, which will be detected by theDoppler flow meter, and the aspiration system (vacuum or flow pumpbased) can respond accordingly.

In yet another approach to detect the occurrence of an occlusion is toutilize a technique known in the art as thermodilution, which is themeasurement of rate of fluid flow in a fluid circuit based on the changein temperature of the fluid from one point of the circuit to anotherdownstream. Turning to FIG. 7, another vacuum-based aspiration system1005 is shown having the same parts as aspiration system 1000 exceptinstead of pressure sensors 1300 and 1350, temperature sensors 1500/1600are used. In this embodiment, a first temperature sensor 1500 formeasuring fluid is coupled to the aspiration line 1110 towards thedistal section of the line 1110, and a second temperature sensor 1600 iscoupled downstream of the first sensor 1500 towards the proximal sectionof the line 1110. The change in temperature, ΔT, in the fluid as read bythe sensors 1500/1600 will correlate with the rate of fluid flow, as oneof ordinary skill in the art would appreciate, e.g., a higher rate offluid flow will cause a smaller change in temperature compared to alower rate of fluid flow. In another embodiment (not shown), instead ofa first temperature sensor 1500, a fluid of a known starting temperatureis injected into the aspiration line 1110. Again, a ΔT is measured usingthe second temperature sensor 1600. In yet another embodiment, insteadof or in addition to a first temperature sensor 1500, a heating element1490 is used to heat the fluid within the line 1110 to a desiredtemperature. Again, ΔT is measured using the second temperature sensor1600.

Cataract Removal System

Turning to FIG. 8, a functional block diagram of a phacoemulsificationsystem in accordance with a preferred embodiment is shown. The system2000 includes a control unit 2102 and a handpiece 2104 operably coupledtogether. The handpiece 2104 may include a needle (not shown) forinsertion into an eye B and a vibrating unit (not shown) that isconfigured to ultrasonically vibrate the needle. The vibrating unit,which may include, e.g., a piezoelectric crystal, vibrates the needleaccording to one or more parameters, such as frequency, pulse width,shape, size, duty cycle, amplitude, and so on. The handpiece 2104provides power, P, irrigation fluid, F, from an irrigation fluid (“IF”)source 2128, and an aspiration line A.

The control unit 2102 includes a vacuum-based pump 2112 operativecoupled to aspiration line A, first and second vacuum sensors 2250/2260in between the pump 2112 and the handpiece 2104, and a flow restrictor2270 (which is preferably variable) in between the vacuum sensors2250/2260. The control unit 2102 further includes a microprocessorcomputer 2110 which is operably connected to and controls the variousother elements of the system, such as the vacuum-based pump 2112, avacuum level controller 2200, a pulsed ultrasonic power source 2114, aflow restrictor controller 2116 (which controls the volumetric flow rateof the aspiration line A by controlling the flow restrictor 2270 asdescribed above), and au ultrasonic power level controller 2118 inaccordance with algorithms described in the Claus application referencedabove. A pressure differential ΔP sensor 2120 provides an input to thecomputer 2110 representing the pressure differential between the firstand second vacuum sensors 2250/2260. Venting may be provided by a vent2122. The control unit 2102 may also include a phase detector 2124 forproviding an input to the computer 2110 that represents a phase shiftbetween a sine wave representation of the voltage applied to thehandpiece 2104 and the resultant current into the handpiece 2104. Thefunctional representation of the system 2000 also includes a system bus2126 to enable the various elements to be operably in communication witheach other.

Dual Pump System

Turning to FIG. 9, an irrigation/aspiration cassette 3000 (preferablydisposable) is shown for use in a surgical system, such as aphacoemulsification system. The irrigation/aspiration cassette 3000includes an irrigation source 3100 operatively coupled to a handpiece3200 via an irrigation line. An irrigation valve 3150 controls theirrigation source 3100. The handpiece 3200 is further coupled to theaspiration portion of the cassette 3000 having both a vacuum-based pump3500 and a flow pump 3300. The operation of one or the other iscontrolled by a selector valve 3250. When the selector valve 3250 isclosed, then the flow pump 3300, which is a first peristaltic pump 3300is the present embodiment, aspirations the fluid from the handpiece3200. The flow pump 3300 pushes the fluid to a holding tank 3450, whichis then drained to a collection bag 3600 by a second peristaltic pump. Avacuum sensor 3750 communicatively coupled to a computer system notshown) is utilized between the flow pump 3300 and the handpiece 3200 todetect any change in vacuum level, which can indicate a possibleocclusion.

When the selector valve 3250 is open, then the fluid flows through thecircuit controlled by the vacuum-based pump 3500, which creates anair-vacuum in the holding tank 3450 that sucks the fluid from thehandpiece 3200. The aspiration portion of the cassette 3000 furtherincludes an air filter 3350 and a vent valve 3400, which are utilized bythe flow pump 3300 and the vacuum-based pump 3500. Further included inthe circuit controlled by the vacuum-pump 3500 is a flow restrictor 3650(preferably variable) and an air transducer 3700 operatively located inbetween the holding tank 3450 and the vacuum pump 3500. The flowrestrictor 3650 enables direct control of the volumetric flow rate, asexplained above, and the pressure differential, ΔP, can be measured byusing both the air-coupled vacuum transducer 3700 and the fluid-coupledvacuum sensor 3750 also used by the flow pump 3300 when in operation.The aspiration/irrigation cassette 3000 shown in FIG. 8 provides thesurgeon with a choice between a vacuum-based pump or a flow pump withina single surgical system, such as a phacoemulsification system. Further,the cassette 3000 allows the flow pump 3300 and the vacuum-based pump3500 to share a common fluid circuit, which can reduce the manufacturingcosts and the volume/length of the aspiration pathway is reduced.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Forexample, the reader is to understand that the specific ordering andcombination of process actions described herein is merely illustrative,and the invention may appropriately be performed using different oradditional process actions, or a different combination or ordering ofprocess actions. For example, this invention is particularly suited forapplications involving medical systems, but can be used beyond medicalsystems in general. As a further example, each feature of one embodimentcan be mixed and matched with other features shown in other embodiments.Additionally and obviously, features may be added or subtracted asdesired. Accordingly, the invention is not to be restricted except inlight of the attached claims and their equivalents.

1. An aspiration system comprising: an aspiration line having distal andproximal ends and an aspiration port defined in the distal end; a fluidtransport device operatively coupled to the proximal end of theaspiration line; a flow restrictor operatively coupled to the aspirationline in between the fluid transport device and the aspiration port; afirst vacuum sensor operatively coupled to the aspiration line betweenthe port and the restrictor; a second vacuum sensor operatively coupledto the aspiration line between the restrictor and the fluid transportdevice; and a computer program product that includes a computer-usablemedium having a sequence of instructions which, when executed by aprocessor, causes said processor to execute a process to measure apressure differential equal to a difference in a first pressure of thefirst vacuum sensor and a second pressure of the second vacuum sensor.2. The system of claim 1, wherein the aspiration system is configured tobe coupled to a phacoemulsification system.
 3. The system of claim 1,wherein the fluid transport device is a vacuum-based pump.
 4. The systemof claim 1, wherein one of the first and second vacuum sensors includesan air transducer.
 5. The system of claim 1, wherein the flow restrictoris a variable flow restrictor.
 6. The system of claim 5, wherein theaspiration line includes a deformable segment operatively coupled to thevariable flow restrictor, and the variable flow restrictor comprises: aplunger element; and an actuator coupled to the plunger element andconfigured to deform the deformable segment via the plunger element. 7.The system of claim 5, further comprising: a computer program productthat includes a computer-usable medium having a sequence of instructionswhich, when executed by a processor, causes said processor to controlthe variable flow restrictor.
 8. The system of claim 1, furthercomprising a controller operatively coupled to the computer programproduct and configured to: control a system parameter to be greater thanor equal to the lower value or to be less than or equal to an uppervalue; sense an occlusion in the aspiration line based on the pressuredifferential between the first and second vacuum sensors; determine aduration of occlusion; and control a system parameter based at least inpart on at least one of (1) the sensing of an occlusion and (2) theduration of occlusion. 9.-28. (canceled)
 29. The system of claim 1,further comprising a controller operatively coupled to the computerprogram product and configured to sense a drop in a controlled systemparameter below a lower threshold and, in response to the drop, controlthe controlled system parameter to be less than or equal to an uppervalue.
 30. The system of claim 1, further comprising a controlleroperatively coupled to the computer program and configured to sense arise in a controlled system parameter above an upper threshold and, inresponse to the rise, control the controlled system parameter to begreater than or equal to a lower value.